To a microwave safe bowl add the chocolate. Microwave on high in 20 second increments, stirring each time until melted. Stir in the espresso powder.
Add the heavy cream to a stand mixer or mixing bowl using a hand whisk, beat until it starts to thicken. Add the vanilla and melted chocolate, continue to beat until light and fluffy and well mixed. Set aside.
Add the white chocolate to a microwave safe bowl and microwave on high for 20 second increments, stirring each time until melted. Pour into a small ziptop bag and seal. Cut a tiny corner of the bag and drizzle onto the inside of glasses. Skip this step if not using white chocolate drizzle.
Divide the mousse between serving bowls and refrigerate for 1-4 hours until set.